chemistry : a yellow chemical element that has a strong, unpleasant odor when it is burned and that is used in making paper, gunpowder, medicine, etc.
Full Definition of SULFUR
: a nonmetallic element that occurs either free or combined especially in sulfides and sulfates, is a constituent of proteins, exists in several allotropic forms including yellow orthorhombic crystals, resembles oxygen chemically but is less active and more acidic, and is used especially in the chemical and paper industries, in rubber vulcanization, and in medicine for treating skin diseases — see element table
The spelling sulfur predominates in United States technical usage, while both sulfur and sulphur are common in general usage. British usage tends to favor sulphur for all applications. The same pattern is seen in most of the words derived from sulfur.